$num = gc 8.txt

function product() {
	BEGIN {
		[int]$prod = 1
	}
	PROCESS {
		#write-host 'prod ' $prod
		$prod *= [int][string]$_
		#write-host $_ $prod
	}
	END {
		$prod
	}	
}

$max = 0
for ($i=0; $i -lt $num.length - 5; $i++) {
	$curr = $num[$($i)..$($i+4)] | product
	if ($curr -gt $max) {
		$max = $curr
	}
}
$max